M-. on an accessor to a slot of a condition catapults you to
It is defined in condition.lisp and is later redefined in
target-condition.lisp once CLOS has been set up.
It's not difficult to pass %DEFINE-CONDITION's source location to the
late version, and then create methods using this source location.
I do not know how to do the same for the early version, though. There's
probably a way to frob a function's stored TLF number, but I haven't
been able to figure out how (I could not even determine the code place
where this information is stored.)
There may also be better alternatives viable.